Method for applying metallic coatings to ceramic bodies



H. VATTER Dec. 6, 193 8.

METHOD FOR APPLYING METALLIC COATINGS TO CERAMIC BODIES Filed June 11, 19.36

Myza

Patented Dec. 6, 1938 UNITED STATES PATENT OFFICE METHOD FOR APPLYING METALLIC COAT- INGS TO CERAMIC BODIES Hans Vatter, Berlin-Charlottenburg,

Germany,

Application June 11, 1936, Serial No. 84,669 In Germany June 19, 1935 4 Claims.

My invention relates to a method for applying metallic coatings to ceramic bodies or for soldering such bodies to one another or to metals so as to obtain a gas-tight bond between the two bodies 5 to be soldered.

The metallization of ceramic bodies, 1. e. the application of an intimate metallic layer to such bodies presents great difficulties. This is particularly true if it is required to obtain highly conductive coatings which readily dissipate the heat and whose adhesive force is not impaired by rapid changes in temperature.

It is well known in the art to provide the ceramic bodies at first with a glaze, to fire there upon a thin platinum layer according to the platinum-chloride method and then to reenforce the layer by electroplating it, for instance with copper. This method is, however, relatively expensive and has further the disadvantage that a component of low melting point is employed, viz. the glaze, which easily evaporates when subjected to a high temperature and which when employed in connection with vacuum discharge devices may contaminate their contents of gas or impair their vacuum.

Another well-known method of applying conductive layers to ceramic bodies consists in applying the metallic coating by volatillzation of the cathode or in producing with the aid of shellac a slightly conducting graphite layer which may then be electrolytically strengthened.

However, the metallic coatings produced by the above-mentioned methods do not adhere so firmly as to serve as a support for a reliable bond. The

need of such ceramic to ceramic or ceramic to metal bonds of substantially absolute vacuumtightness is becoming more and more general. Thus, for instance, in the case of spark plugs consisting of sintered aluminium oxide it is necessary to provide a coherent bond between the ceramic and the metallic parts. Particularly in vacuum discharge devices in which a relatively absolute vacuum is to be maintained the formation of gas-tight bonds is imperative, insofar as in the manufacture of vacuum discharge devices besides metals also ceramic materials are now being employed. It is, therefore, necessary that such bonds be highly adhesive and of an absolute vacuum-tightness. It is well known to solder bodies of ceramic material to one another or to metal bodies by soft soldering, i. e. by a method which consists in applying at first a thin coating oi! silver, platinum or copper to the 5 ceramic material and in strengthening this layer by electroplating it with a soft solder according to one of the known methods.

The known methods, however, present the further drawback in that it is impossible to obtain a reliable joint by the use of hard solders. For this reason, the bonds produced according to these methods cannot, for instance, withstand the temperatures to which the vacuum discharge devices must be ordinarily subjected when degasifying the metallic parts thereof. Further- 10 more, such bonds frequently present leakages owing to the porosity of the galvanic coatings and cannot, therefore, be employed for vacuum discharge devices in which an absolutely vacuum-tight bond is indispensable. 15

According to the invention a highly refractory base metal such as, for instance, iron, chromium, tungsten or nickel or an alloy of such a metal is applied to the'ceramic body in the form of finely divided powder and then the whole is heated in 20 an atmosphere free of oxygen until the metal sinters. To enhance the adhesive force of the metallic powder on the ceramic material before sintering the same it is advisable to stir the powder with a liquid. To this end organic bind- 25 ing agents well known in the art may be employed.

In applying a metallic coating to ceramic bodies any base metal capable of being sintered may be employed, especially highly refractory metals 30 such as, for instance, iron, chromium, nickel, tungsten or the like. They may be singly employed or alloyed with one another or with other metals. The essential point is that the melting point of the metallic coating is higher than the 35 melting point of the solder to be employed.

When sintering the metallic body the temperature must be accurately maintained. With a temperature of 1100 C. the sintering when employing, for instance, iron is still insuflicient or it 40 requires at least such a long time as to render the method expensive and complicated. On the other hand, with a temperature of above 1280 0. instead of a uniformly compact iron coating a black spotted metallic coating is formed which 45 does not permit, for instance, a soldering with metals. When using pulverized iron it is, therefore, preferable to carry out the method according to the invention within a temperature range of 1220-1260 C. 50

The metallic coatings made according to the invention may be used for various purposes. Thus, for instance, in the case of vacuum discharge devices electrodes, such as anodes, may be soldered according to the invention to the inner walls of ceramic vessels. Experiments have shown that such coatings are highly heat-conductive. It is also possible according to the same method to provide the surfaces of the ceramic discharge vessels with coatings which prevent the formation of charges along the walls.

According to the novel method resistances, particularly vacuum resistances, may be manufactured, if the coating to be applied is sumciently thick. This may be accomplished by molding the metallic powder paste-app1ied to the inner or outer surfaces of a ceramic body having, for instance, a cylindrical shape-directly into a spiral or thread by means of a molding device and then heating the coating thus molded to the sintering temperature according to the method of the invention. Also the entire ceramic body forming part of the resistance may be provided with a coating by immersing it in the paste, heating it to the sintering temperature and then forming the coating thus obtained, for instance, into a spiral by a grinding operation.

By substituting a spiral consisting of metallic powder which is heated on a ceramic body to the sintering temperature for the usual iron wire the method according to the invention may also be used for the manufacture of resistances having the characteristic of the so-called hydrogen-iron resistances.

Finally the novel method according to the invention may be also used to a great extent in connection with insulators.

The application of a highly refractory base metal has further the advantage that a hard solder may be employed for joining such metallized bodies with one another or with other bodies. By the methods hitherto employed for metallizing ceramic bodies with gold, silver, platinum or copper this is not possible, since the hard solders available form with such metals coatings which are not sufficiently resistant owing to their low melting temperature. Only a coating of a highly refractory base metal produces a durable bond which is also sufliciently resistant to high temperatures. Furthermore, such a bond has a great adhesive force and is vacuum-tight and, therefore, meets all requirements in the manufacture of vacuum-discharge devices.

In forming bonds between metallized ceramic bodies it is preferable to use a solder as ductile as possible which forms an intimate joint with the metallic coatings even if the coemcients of expansion of the solder and of the ceramic body differ from each other. If, however, materials are employed for the bonds, the coefficients of expansion of which have the same or nearly the same value, the particular ductility of the solder may, of course, be dispensed with.

It has been found that it suffices to apply the metallic solder to the joint in a solid form; for instance, in the form of a ribbon, rod or ring, in order that the solder when heated may spread over all surfaces of the metallic coating, thus forming the desired vacuum-tight bond. Both the sintering and the soldering process may be effected in the open air if materials are employed for the metallic layer which neither oxidize her experience any detrimental changes when exposed to the open air. Otherwise the heattreatment is effected under vacuum or also in a nonoxidizing or reducing atmosphere.

If as above described the hard solder is applied to the joint in the form of a ribbon, rod or ring the sintering and soldering process may be combined in a single operation.

In the accompanying drawing are shown some applications of the invention in diagrammatic form.

In Fig. 1, I and 2 denote ceramic cylinders to be joined at the points as indicated at 3. I are the layers consisting of iron which are sintcred at the points to be soldered, for instance, at a temperature of 1200 C. 5 is a ring of hard solder which during the heating process spreads over the entire surface of the metallic layer 4, thus forming a vacuum-tight bond between the parts I and 2.

In Fig. 2 is shown a fuse for lightning arresters frequently employed in telegraphy and telephony. The body I consists of ceramic material. 6 and l are metallic caps which are to be joined with the central portion so as to form a vacuum-tight bond. 4 denotes the metallic layer and I the ring of hard solder. Also in this case a coherent and vacuum-tight bond is produced by causing the hard solder during the heating process to flow along the entire surfaces to be united and t0 penetrate the joint so as to form a vacuum-tight bond therebetween.

Fig. 3 illustrates a spark plug, the part 8 of which, consisting of sintered aluminium oxide, is to be soldered at the joint as indicated at 3 with the metallic part 9. Also in this case 4 denotes the metallic layer sintered on the part 8, whereas 5 denotes the circular metallic solder. The soldering process is effected in the above-described manner.

In Fig. 4, l0 denotes a portion of an electric discharge vessel made of ceramic material on which a vacuum-tight metallic cap H is to be placed. The metallic layer and the ring of metallic solder are denoted by I and 5 respectively.

The method according to the invention for metallizing or soldering ceramic bodies may be employed for various other purposes than those shown in the drawing. It is particularly suitable in all cases in which a durable and vacuumtight bond is required. By the method according to the invention, for instance, a reliable joint between the individual parts of vacuum discharge devices may be obtained which are mounted either on vehicles or are subjected when in operation to great mechanical stresses.

According to the invention also other insulating materials than those above described may be metallized and soldered to one another or to metals.

I claim as my invention:

1. The method of soldering a body having a metallic surface to a ceramic body, consisting of applying a metallic coating of iron directly to the ceramic surface of said ceramic body in the form of a finely divided powder, heating the body in an atmosphere inert with respect to the iron at a temperature between 1220 and 1260 C. until the iron sinters to a. solid and gas-tight layer, and soldering said-layer to said first body with hard solder.

2. The method of producing a gas-tight joint between a ceramic body and another body having a metallic surface, comprising the steps of apply ing a metallic coating consisting of a finely divided powder of a metal having a melting point above 1000 C, and being selected from the group consisting of refractory base metals and their alloys and of a liquid organic binding agent directly to said ceramic body, heating said ceramic body in an atmosphere inert with respect to said metal until saidcoating sinters, joining said ceramic body and said other body together at said sintered coating, applying a hard solder to the joint and heating both bodies in an atmosphere inert with respect to said sintered metal and said hard solder until said hard solder melts.

3. The method of joining a body having a me-' tallic surface with a ceramic body having an unglazed surface, comprising the steps of applying to said surface of said ceramic body a finely divided mass consisting of a metal selected from the group consisting of refractory base metals and their alloys and having a melting point above 1000" C., heating said ceramic body in an atmosphere inert with respect to said metal until said metal sinters, joining said two bodies, interposing hard solder in the joint so as to 1111 only part of the interstice between said two bodies, and

heating the joint until the hard solder fuses and penetrates the joint.

4. The method of soldering a body having a metallic surface to a ceramic body having an unglazed surface, comprising the steps of applying to said surface of said ceramic body a coatin consisting of a finely divided metal having a melting point above 1000 C. and selected from the group consisting of refractory base metals and their alloys, heating said ceramic body in an atmosphere inert with respect to said metal at a temperature causing said metal to sinter and to form a gas-tight layer, and soldering said layer to said first body by means of hard solder. 

